The invention relates to a high-speed output driver as generically defined by the preamble to claim 1. Such high-speed output drivers serve in particular to connect two digital circuits via waveguides, in particular 50-ohm lines or 75-ohm lines. They are embodied in CMOS circuits with preferably low supply voltages ( less than 2.5 volts).
One fundamental element of such high-speed output drivers is a differential driver or a single ended driver shown in FIGS. 1 and 2, respectively. In FIG. 1, a differential driver has two standard CMOS inverters with n-channel MOS transistors T1, T3 and complimentary p-channel MOS transistors T2, T4. The two inverters are operated with via input nodes I, In with a differential input voltage Uin. At the node points Q, Qn of the drain terminals of the two inverters, the output voltage Uout is picked up. At the output Q, Qn, a differential load R3 is present, typically of 2xc3x9750 ohms=100 ohms. Such a differential driver makes a push-pull output available.
A single ended driver as in FIG. 2 has a standard CMOS inverter with an n-channel MOS transistor T1 and a p-channel MOS transistor T2, which are connected in series. The CMOS inverter acts on a load R3, typically of 50 ohms, at which a rex Vref is present. The rex Vref corresponds for instance to half the supply voltage (single ended output).
The circuits shown in FIGS. 1 and 2 are basic circuits, which for a specific application are given the requisite properties by way of their dimensioning. In particular, the drain-to-source resistances of the transistors are adjusted to suit the desired application.
One disadvantage of the known CMOS inverters or drivers, however, is that the transistor properties are subject to major fluctuations. These fluctuations are due in particular to temperature fluctuations and to parameter fluctuations in the manufacturing process. The properties of the transistors of the CMOS inverters are thus subject to major fluctuation in the temperature and over the course of the production process. These fluctuations, in the applications described, can range up to 100%.
On the other hand, for the high-speed output driver discussed, the requirement is
1. that it have a constant output level, that is, a relatively constant output current, at an external load such as a 50-ohm line with a terminal resistance, and
2. that the driver have a relatively constant output resistance, to prevent reflections from the source. Then, as much as possible, the driver should have an active load in the form of a switching transistor, since passive loads either increase the current consumption or cannot be used at low voltage.
The above two requirements cannot be achieved with conventional CMOS inverters, because of the aforementioned fluctuations in the transistor properties over the temperature and production process. There is accordingly a need to refine the known CMOS inverters or driver circuits with such inverters in such a way that constant outset conditions, that is, a substantially constant output current and a substantially constant output resistance of the driver, prevail.
In the publication entitled xe2x80x9cItanium(trademark) Processor System Bus Designxe2x80x9d by A. Ilkbahar, S. Venkataraman, H. Muljono, ESSCIRC 2000, Stockholm, Sep. 19-21, 2000, pages 128-131, an output driver circuit for the Intel Itanium(trademark) processor is described, in which an inverter for setting a desired output impedance is discretely connected to parallel inverters. However, this is done with a view to adapting the driver to variable capacitive output loads.
The object of the present invention is to make a high-speed output driver with a CMOS inverter acting as an amplifier for digital signals available which is distinguished by a substantially constant output current and a substantially constant output resistance.
Hence according to the invention, switch means are provided which are connected to sensor elements for detecting parameters of the transistors of the CMOS inverter, and which as a function of the detected parameters control or regulate the output current and the output resistance of the driver to an essentially constant value. The input parameters of the transistors, which are delivered as input signals to the switch means, are in particular the temperature and parameters pertaining to the production process. These parameters are detected via a temperature sensor and process sensor, respectively.
Accordingly, the invention furnishes a controller or regulator which as its input signals receives transistor parameters that are subject to fluctuations, and which on the basis of these input signals controls or regulates the output current and the output resistance of the driver to a substantially constant value.
For control (without feedback) or regulation (with feedback), additional transistors are added either digitally (discretely) or in analog fashion, which change the output properties of the driver and in particular its drain-to-source resistance.
In a preferred feature of the invention, it is accordingly provided that connected to the output of the CMOS inverter is the output of at least one further, additional CMOS inverter. This at least one further CMOS inverter is discretely connected to or disconnected from corresponding switches via digital control signals. Via the addition or subtraction of additional individual transistors or inverters, the output properties of the driver can be varied as desired. Thus an added inverter for the output makes an increased current (higher output level) available and a reduced output impedance.
The addition of further inverter stages can be done alternatively by controlling the gate terminal or the source terminal.
Preferably, the size of the transistors is weighted and in particular amounts to 1, 2, 4, . . . , 2n times a basic size, so that a fine adjustment of the output properties of the driver can be made.
A control unit which receives analog input signals of a temperature sensor and a process sensor and on the basis of these signals outputs digital control signals to switches for discrete addition or subtraction of the further transistors or CMOS inverters is preferably used as the control means.
In an alternative variant embodiment of the invention, adding of individual transistors for varying the output properties of the driver is done not via a digital controller but via an analog controller. It is provided that the switch means control additional transistors in analog fashion, and at least one additional transistor is connected in series with the n-channel and p-channel MOS transistors of the inverter, so that via the additional transistors, the impedance of the amplifier branches of the inverter can be adjusted.
The added transistors are preferably each operated in the triode range, in which a transistor represents a linear resistance. The resistance additionally made available by the transistor is substantially linearly dependent, in this range, on the voltage applied to the gate terminal.
In a further feature of the invention, not only is the output level controlled but also regulated; that is, work is additionally done with feedback. Regulation is preferably done by the provision of a regulating circuit in the form of a mirror circuit, which regulates the operating point of each added transistor. The elements of the mirror circuit correspond in their size to the elements of the driver circuit, or are scaled by a certain value compared to the driver circuit. By scaling by the factor n, the current consumption can be reduced by the factor 1/n.
The mirror circuit preferably includes an operational amplifier, which regulates the control voltage for the mirror circuit to a value such that the desired output level is established at the output of the mirror circuit and thus also at the output of the driver.
The invention will be described in further detail below in conjunction with the drawing figures and in terms of several exemplary embodiments. Shown are: